Shia scholar Hujjatul Islam Mirza Muhammad Taqi in his book “Saheefatul Abrar” (1/328) narrated:

 بصائر الصفار حدثنا الحسن بن علي ، عن صالح بن سهل، عن أبي عبد الله (عليه السلام) قال : (كنت جالسا عنده فقال ابتداء منه: يا صالح بن سهل ، إن الله جعل بينه وبين الرسول رسولا ، ولم يجعل بينه وبين الإمام رسولا ، قال : قلت : وكيف ذاك؟ قال :جعل بينه وبين الإمام عمودا من نور، ينظر الله به إلى الإمام ، وينظر الإمام إليه ، إذا أراد علم شيء نظر في ذلك النور فعرفه

(From) Basair of Saffar: Narrated to us al-Hasan ibn Ali, from Salih ibn Sahl from Abu Abdullah (alaihi salam) (Salih) said: We were sitting with him, (when) he said starting himself: O Salih ibn Sahl: Allah made messengers between Himself and Prophet, and didn’t made messenger between him and Imam. (Narrator) said: I said: And how is that? (Imam) said: He made between Himself and Imam column from light, Allah looks to Imam by it, and Imam looks to Him. If he needs anything from knowledge he looks at this light (and after this) he knows it.